Rate constants for alkaline hydrolysis

Yukawa and coworkers (1972)84 determined a0 values from the rate constants for alkaline hydrolysis of m- and p-substituted-benzyl benzoates in 70% (v/v) aqueous acetone at 25 °C. ap° values for SOMe and S02Me were found to be 0.573 and 0.749 respectively. These were compared with 0.564 and 0.721, respectively, for values determined from the rate constants of alkaline hydrolysis of substituted ethyl benzoates in 85% aqueous ethanol. From these values there is no evidence for any — R cross-conjugative effect of SOMe as a substituent in the benzoate moiety, which is eliminated when it is in the benzyl. However, both the values for SOMe are substantially higher than most of the ap values for SOMe which we have surveyed previously. For S02Me the order ap° > if significant,... 

Fig. 12 Dependence of the rate constant for alkaline hydrolysis for three amides [58] (x = 1, y = 2 x = 2, y = 1 and x = y = 2), and [59] on three different geometrical parameters the twist-torsional angle r (A), and the out-of-plane bending at the amide carbonyl (O) and nitrogen ( ) centres. [The geometrical...

It is well-established that the reactivity of aromatic nitrogen mustards in hydrolysis reactions correlates positively with the degree of electron release to the nitrogen (7). The rate constants for alkaline hydrolysis of a series of aniline mustards correlate well with a, as shown by Equation 1 (equation 23 of ref. 8). 

One of the most comprehensive studies has been carried out by Bruice et al. [19] who studied the rate of solvolysis of neutral, positively and negatively charged esters when incorporated into non-functional and functional micelles of neutral, positive and negative charges. The second-order rate constants for alkaline hydrolysis, /cqh [0H ] were found to decrease with increasing concentration of surfactant for all cases studied. The association of the esters with non-nucleophilic micelles must either decrease the availability of the esters to OH attack or provide a less favourable medium for the hydrolysis reaction to occur. This is another circumvention of the simple electrostatic rules as the kinetic effect seems to have nothing to do with the concentration or restriction of access of the hydroxyl ions in the Stern layer of the micelles. Presumably the labile ester bond is not positioned near the surface of these micelles, but the molecules are oriented as shown in Fig. 11.2. 

Example 4.3. The p value for alkaline saponification of methyl esters of substituted benzoic acids is 2.38, and the rate constant for saponification of methyl benzoate under the conditions of interest is 2 x 10 s . Calculate the rate constant for the hydrolysis... 

Alkali diphosphates are practically stable in alkaline and neutral solutions, but they are more and more rapidly hydrolyzed with decreasing pH. Over the whole pH range, however, they are more stable, under the same conditions, than all other condensed phosphates. Hydrolysis is accelerated by an increasing temperature or ionic strength (44, 846). The table below shows the dependence of the rate constant for the hydrolysis on pH at... 

Interest in the kinetics of alkaline hydrolysis of esters in DMSO + water mixtures was stimulated by the observation that the rate constant often increased gradually as x2 increased. This is observed, for example, in the alkaline hydrolysis of ethyl acetate. For higher esters, e.g. ethyl p-nitrobenzoate, the rate constant drops slightly at low x2 but then rises again until k/k x2 = 0) > 1 (Tommila, 1964). The rate of alkaline hydrolysis of esters of benzoic acid is accelerated when DMSO is added (Tommila and Palenius, 1963), as also is the rate of alkaline hydrolysis of 2,4-dinitrofluorobenzene. In the latter case the effect is less dramatic because the rate constant for spontaneous hydrolysis also increases (Murto and Hiiro, 1964). The rate constants also increase when DMSO is added to aqueous solution for reactions between hydroxide ions and benzyl chloride (Tommila... 

Several features of the above studies were reinvestigated in a detailed kinetic study of the copper(II) complexes of glycine methyl ester and phenylalanine ethyl ester in glycine buffer at pH 7.3 (26). Glycine was selected as a buffer in this study in order that a small increase in the glycine concentration caused by the hydrolysis reaction would not increase the concentration of copper(II) complexes to a significant extent. It was found that the rate constant for the hydrolysis of the copper(II) complex of DL-phenylalanine ethyl ester was 106 times greater than the rate constant obtained for the alkaline hydrolysis of the free ester (25). 

Holmquist and Bruice [110] correlated the alkaline hydrolysis of 2-nitrophenyl substituted acetates versus the water rate constant for the hydrolysis (Fig. 25). The slope of the excellent correlation is just less than unity indicating that the mechanisms have similar electronic and steric requirements. However, 2-nitrophenyl ethyl malonate, cyanoacetate and dimethylsulphonioacetate esters react faster with hydroxide ion than is expected from the relationship and are therefore judged to pass through different mechanisms (Eqns. 117 and 118). 

The value of the Hammett equation in estimating reaction rates is illustrated as follows. Suppose it is required to know the rate of alkaline hydrolysis of p-nitrobenzyl acetate in 56% acetone at 25°C. p for the general reaction is +0.760 and the rate constant for the hydrolysis of benzyl acetate is 6.995 X 10 1 mole" sec". The substituent constant for p-nitro is +0.778. The required rate constant is obtained from the equation ... 

As it is already stressed in the Introduction, the fulfillment of thermodynamic requirements is a necessary but not sufficient prerequisite for a polymerization to occur. In general, the Thermodynamic polymerizability cannot be taken as a direct measure of monomer reactivity. For instance, the rate constants of alkaline hydrolysis of y-BL and CL at comparable conditions are close to each other (1.5 x 10" and 2.6 x lO lmol s" respectively) although the corresponding ring strains differ considerably (cf. Table 1). 

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

These are pseudo-first-order rate constants for the alkaline hydrolysis of ethyl / -nitrobenzoate at 25°C. 

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

Quantitative structure-reactivity analysis is one of the most powerful tools for elucidating the mechanisms of organic reactions. In the earliest study, Van Etten et al. 71) analyzed the pseudo-first-order rate constants for the alkaline hydrolysis of a variety of substituted phenyl acetates in the absence and in the presence of cyclodextrin. The... 

It is the reaction characterized by fc2(lim) that exhibits the specificity toward the position of the phenyl group substituent, and is responsible for the accelerated rates of appearance of phenol. The rate-limiting step of the overall reaction, however, is the hydrolysis of the acyl-cycloamylose. The overall reaction, then, will be catalytic only if k3 exceeds the rate constant for the alkaline hydrolysis of a particular ester. This situation is true only for highly unreactive esters. If, therefore, the cycloamyloses are to be uti-... 

The reaction of cycloheptaamylose with diaryl carbonates and with diaryl methylphosphonates provides a system in which a carboxylic acid derivative can be directly compared with a structurally analogous organo-phosphorus compound (Brass and Bender, 1972). The alkaline hydrolysis of these materials proceeds in twro steps, each of which is associated with the appearance of one mole of phenol (Scheme Y). The relative rates of the two steps, however, are reversed. Whereas the alkaline hydrolysis of carbonate diesters proceeds with the release of two moles of phenol in a first-order process (kh > fca), the hydrolysis of methylphosphonate diesters proceeds with the release of only one mole of phenol to produce a relatively stable aryl methylphosphonate intermediate (fca > kb), In contrast, kinetically identical pathways are observed for the reaction of cycloheptaamylose with these different substrates—in both cases, two moles of phenol are released in a first-order process.3 Maximal catalytic rate constants for the appearance of phenol are presented in Table XI. Unlike the reaction of cycloheptaamylose with m- and with p-nitrophenyl methylphosphonate discussed earlier, the reaction of cycloheptaamylose with diaryl methylphosphonates... 

An estimate of the rate enhancement associated with the intramolecular phosphorylation can be made by using isopropyl p-nitrophenyl methyl-phosphonate as a model for the covalent intermediate formed in the initial step of the reaction of cycloheptaamylose with bis (p-nitrophenyl) me thy 1-phosphonate. The first-order rate constant for the alkaline hydrolysis of isopropyl p-nitrophenyl methylphosphonate at pH 9.86 can be obtained from the data of van Hooidonk and Groos (1970) kun = 1.4 X 10-5 sec-1. This value may be compared with the maximal rate constant for the reaction of cycloheptaamylose with bis(p-nitrophenyl) methylphosphonate— k2 = 1.59 X 10-1 sec-1 at pH 9.86—which must be a minimal value for the rate of the intramolecular phosphorylation. This comparison implies a kinetic acceleration of at least 104 which is similar to rate enhancements associated with the formation of cyclic phosphates from nucleoside phosphate diesters. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

Yanovskaya and coworkers55 have reported rate constants for the alkaline hydrolysis of ethyl lruns,trans-4 -substi tuted 5-phenyl-2,4-pentadienoates in 60% aqueous dioxan giving the values X, log t, H, -2.60 Cl, -2.23 Br, -2.31 N02, -1.83 OMe, -2.61 NMe2, —3.00. Correlation with the CR equation gave as the best regression equation ... 

Chemical/Physical. Hydrolyzes in water to o-phthalic acid (via the intermediate 2-ethyl-hexyl hydrogen phthalate) and 2-ethylhexyl alcohol (Kollig, 1993 Wolfe et al., 1980). Although no pH value was given, the reported hydrolysis rate constant under alkaline conditions is 1,400/M-yr (Ellington et al., 1993 Kollig, 1993). A second-order rate constant of 1.1 x 10 /M-sec was reported for the hydrolysis bis(2-ethylhexyl) phthalate at 30 °C and pH 8 (Wolfe et al., 1980). 

The hydrolysis of pesticides which are sorbed to sterilized natural sediments has been investigated in aqueous systems at acid, neutral and alkaline pH s. The results show that the rate constants of pH independent ("neutral") hydrolyses are the same within experimental uncertainties as the corresponding rate constants for dissolved aqueous phase pesticides. Base-catalyzed rates, on the other hand, are substantially retarded by sorption and acid-catalyzed rates are substantially enhanced. A large body of evidence will be presented which substantiates these conclusions for a variety of pesticide types sorbed to several well-characterized sediments. The significance of our results for the evaluation of the effects of sorption on the degradation of pesticides in waste treatment systems and natural water bodies will also be discussed. 

Experimental and Calculated Values of the Rate Constants for the Alkaline Hydrolysis of Chlorpyrifos in Sediment/Water Systems ... 

These conclusions have several implications for pesticide waste disposal considerations. For incidental or accidental disposal of pesticides in natural aquatic systems, the results suggest that model calculations using aqueous solution values for abiotic neutral hydrolysis rate constants can be used without regard to sorption to sediments. For alkaline hydrolysis, on the other hand, models must explicitly include sorption phenomena and the correspond ng rate reductions in order to accurately predict hydrolytic degadation. 

Ethyl chloroacetate, C4H7CIO2, is used as a solvent and in the synthesis of intermediate dye chemicals. The effluent from a dye synthesis plant is discharged into a wetland, with a pH of 7.3. At this pH, the alkaline hydrolysis rate constant is 1.56 M s . If the alkalinity of the wetland is close to constant, compared with the concentration change of the ethyl chloroacetate, at 0.002 M, what would be the first-order rate constant for ethyl chloroacetate degradation ... 

The serine group which becomes phosphorylated does not appear to possess any marked nucleophilic reactivity, nor is there any evidence that a histidine group participates as a general acid-general base catalyst. Rate constants for the nonenzymic hydrolysis of alkyl and aryl phosphate monoanions at 25° are in the range HP7 to 10-9 sec-1 (167), while the comparable alkaline phosphatase-catalyzed values (in this case they refer to dianions) are in the range 102 to 103 sec-1. Thus one has to account for a rate enhancement factor of 109 to 1012. Moreover, the... 

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